The general idea of a "plain" multiboot system (BIOS only) is that there is one "boot hard disk" or "first hard disk", the firmware (BIOS) loads at boot time (through the MBR and PBR, etc.) a bootmanager/OS loader (still residing on the "first hard disk" ) which in turn loads the operating system (that can reside on another hard disk alright).

With UEFI the MBR code is ignored and the firmware passes control to either a loader registered in the NVRam of the firmware or to a loader within a partition (needs normally to be FAT formatted, need a particular type if on GPT) with a "default" position/name.

I don't understand the "boot OS1 from HDD2".

If "HDD2" is GPT it won't ever boot from BIOS (unless you use a BIOS on GPT method, hybrid MBR, etc.) but even then it needs to be first disk in BIOS boot order (or however you need an intermediate step, like grub4dos to remap the disks in BIOS).

Can you explain what is the actual reason why you want to boot a "same" (or clone) Windows 8.1 install in BIOS and UEFI?

I mean, as soon as the system is actually booted, the OS will behave the same, so what is the need for this?

Yes, a volume is always a volume, and it corresponds to what on MBR is usually a primary partition but that can also be a logical volume inside extended and to what on GPT is always a primary partition.

Technically there is a small difference between volume and partition on NTFS volumes (on both MBR and GPT style disks), because the partition is actually one single sector larger than the volume to allow a copy of the first sector of the VBR (while on all other filesystems the partition and the volume are exactly the same size).

The boot sequence is very different between MBR and GPT (actually between BIOS and UEFI)

UEFI->protective MBR (data only, i.e. first entry in partition table spanning the WHOLE disk)->GPT table->a loader BOOTMGR.EFI or bootmgfw.efi or bootx64.efi either in \boot\efi\ or \EFI\Microsoft\boot\ (or in both places) of the ESP partition (that is normally FAT formatted and NOT mounted to a drive letter)->choice in \efi\microsoft\boot\BCD on the ESP partition ->\windows\system32\Winload.efi on *any* volume BUT the ESP one->Windows install on that volume

There already solutions (with a few limitations) to have a GPT style disk boot from BIOS (besides from UEFI), aka "BIOS on GPT" , but until I don't understand what you are after (final goal) cannot say if it suitable to your question.

Also this:

- Is there an easy way to quickly change/ switch a Windows OS back & forth from a BIOS < to > UEFI booting?

is not a clear question, sure there are ways, whether they are easy is to be seen.

There is BIOS on GPT, there are Hybrid MBR's (with the usual 2.2 Tb limit), there is the possibility of re-writing the MBR (also with the usual 2.2 Tb limit), it depends on what you are after which solution may be suitable (or more suitable)

Is there an easy way to Switch a Windows OS between EFI <> Legacy booting modes on a HDD without having to go through an extensive HDD type conversion?

In general, yes (or maybe no):

Also this:

- Is there an easy way to quickly change/ switch a Windows OS back & forth from a BIOS < to > UEFI booting?

is not a clear question, sure there are ways, whether they are easy is to be seen.
There is BIOS on GPT, there are Hybrid MBR's (with the usual 2.2 Tb limit), there is the possibility of re-writing the MBR (also with the usual 2.2 Tb limit), it depends on what you are after which solution may be suitable (or more suitable)

JFYI, in your topic title you talk of a single HDD (HDD is a common acronym for Hard Disk Drive), then you talk of two HDD's, and you ignored the questions I asked to try and understand what you are after, nor you manage to provide a clearer description of what you have, what you want to do, etc.

JFYI, in your topic title you talk of a single HDD (HDD is a common acronym for Hard Disk Drive), then you talk of two HDD's, and you ignored the questions I asked to try and understand what you are after, nor you manage to provide a clearer description of what you have, what you want to do, etc.

EFI Boot Entry: How do I do that? Using bcdboot? or EasyUEFI/ BCD type of tool? Recommendation?

How I cloned it? (Using Paragon HDM > Copy Partition Wizard).

Do I have to make any changes/ adjustments to the "cloned" Windows ?

Yes, if you didn't use a "specific cloning tool", surely you need to change the \Dosdevices drive letter assignments in the Registry, so that the partition where the "cloned" Windows has been put takes the same drive letter (I have to presume the "C:") it had originally.

And yes again, BCDboot might do - though I cannot say if using it might affect the "current" Windows install - I would rather use BCDedit.

Misty has written a very good guide about the BCD and how to modify its settings:

i know this is late, but, this is totally possible. in fact i can boot my windows 10 in both uefi and cmos mode from the same partition. uefi can be booted from mbr disk as long as it is the correct arch of the cpu and the first partition is fat32 and active.

in OP's situation, all that has to be done is clone the preloaded os in uefi mode from hdd2 to the mbr disk and not fool with the gpt disk at all. make 3 partitions on mbr disk, one fat32 and active, the other two ntfs for each windows system, windows from uefi hdd2 to second partition (first ntfs part), windows from mbr disk to third partition (second ntfs part), bcdboot files from the uefi windows install to the first partition on the hdd (fat32 part) using the /all switch for all firmwares. then use your visual bcd editor of choice to make sure the efi bcd (efi\microsoft\boot\bcd) is loading winload.efi from the first ntfs partition, then make sure the standard bcd store (boot\bcd) points to winload.exe of the second ntfs partition.

keep your bios boot mode in uefi in the dell bios config, and it will boot to uefi by default if you did it correctly, then to boot in cmos just press f12 for the dell boot menu and boot the legacy hdd option and it will boot the second install in cmos mode.

easy peasy. i have a how-to here that i copied from my original thread at mdlforums that explains how this is easily done, as i think i was the first person to discover windows will boot in uefi mode from an mbr disk without any issue.

A caveat: if you are booting Windows in UEFI on MBR, the partition that C drive is on must be primary. I tested with a logical in an extended, it would not boot, just hangs on the loading screen forever. Other partitions can be logical.

So in your case: I would set up:
1. EFI system partition, formatted as FAT32
2. Partition for Windows in legacy
3. Partition for Windows in UEFI
4. Extended partition with logicals within

Install Windows from a legacy booted USB in one partition, then reboot the USB in UEFI and install again into the other partition. In legacy mode use bcdboot to generate boot files, and bootrec to generate an MBR. For UEFI only bcdboot is necessary. Make sure to specify the right drive letters in each case. And be sure to create the boot files for each install while the USB is in the right mode. You can use the EFI system partition as the boot partition for both installs.